Method for manufacturing silicon on sapphire wafer

ABSTRACT

The present invention provides an SOS wafer comprising a non-transparent polysilicon layer provided on a back surface of a sapphire substrate, a silicon nitride layer which protects the polysilicon layer, and a stress relaxing film which cancels stress produced in the silicon nitride layer, wherein the silicon nitride layer and the stress relaxing film are provided on the back surface side.

This is a Divisional of U.S. application Ser. No. 11/377,394, filed onMar. 17, 2006, and allowed on Apr. 6, 2009, the subject matter of whichis incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates to an SOS (Silicon On Sapphire) wafercorresponding to one wafer for manufacturing a semiconductor device, andits manufacturing method.

In a conventional SOS wafer, a silicon layer and an oxide layer arelaminated over the front surface of a sapphire substrate of the SOSwafer. A polysilicon layer having non-transparency and an electricalcharacteristic and a silicon nitride layer which protects thepolysilicon layer are provided over the back surface of the sapphiresubstrate, thereby enabling optical and electrical detection of the SOSwafer.

Such an SOS wafer is formed by a conventional manufacturing method shownin FIG. 9.

The conventional method for manufacturing the SOS wafer will beexplained below in accordance with processes or process steps indicatedby PZ using FIG. 9. Incidentally, FIG. 9 shows partial sections of theSOS wafer.

In PZ1, an SOS wafer 5 is prepared in which a silicon layer 3 and anoxide layer 4 comprised of silicon dioxide are laminated over a frontsurface 2 of a sapphire substrate 1.

Such a normal SOS wafer 5 is of a circular wafer having a diameter of125 mm. The thickness of the sapphire substrate 1 is 640 μm, thethickness of the silicon layer 3 is 1000 Å, and the thickness of theoxide layer 4 is about 3600 Å.

Incidentally, the diameter and shape of the SOS wafer 5 are not limitedto the above. The SOS wafer 5 may be a wafer having other dimensions andshape.

In PZ2, a 2.3 μm-thick polysilicon layer 7 is formed on the back surface6 of the sapphire substrate 1 by using low-pressure chemical vapordeposition (LPCVD). Since the polysilicon layer 7 is grown using thelow-pressure chemical vapor deposition in the present process step, apolysilicon layer 7 is similarly formed even on the front surface 2side.

Incidentally, the drawing indicative of the present process step isshown with its obverse and reverse sides placed in reverse relationshipto the drawing indicative of the process step PZ1. The front surface 2side is not illustrated (the drawings indicative of the process steps ofthe prior art shown below are also same).

In PZ3, ions of an impurity such as phosphorus (P) are implanted in thecorresponding polysilicon layer 7 to form a conductive diffusion region8. This implantation is carried out to obtain conductivity of a siliconwafer in such a manner that an electrical sensor of a silicon waferprocessing apparatus designed so as to detect the silicon wafer, basedon the conductivity of the wafer is capable of detecting the SOS wafer5.

The ions implanted in the present process step are P+ (atomic weight:31), a dose thereof is 1×10¹⁶/cm², and implantation energy is 175 keV.The amount of diffusion of the impurity into the polysilicon layer 7 onthe back surface 6 side is determined depending on the dose of thephosphorus. The implantation energy can also be reduced to 25 keV or so.

After the implantation of the phosphorus, resistivity of the conductivediffusion region 8 is normally set to less than approximately50Ω/square. This is because when the resistivity is set to 50Ω/square ormore, the resistivity becomes excessively high to allow the siliconwafer processing apparatus to detect the SOS wafer 5 as the siliconwafer on the basis of its conductivity.

In PZ4, a 900 Å-thick silicon nitride layer 9 is formed on thepolysilicon layers 7 on the front surface 2 side of the sapphiresubstrate 1 and on the back surface 6 side thereof.

The silicon nitride layer 9 is used as a protection layer for protectingthe polysilicon layers 7 and the conductive diffusion region 8 duringsubsequent processing of the SOS wafer 5.

Incidentally, the thickness of the silicon nitride layer 9 may be set toa range from about 500 Å to about 5000 Å. The thickness of the siliconnitride layer 9 is selected so as to sufficiently protect thepolysilicon layer 7 and the like from subsequent processing applied tothe SOS wafer 5.

The shape of each edge portion 10 of the SOS wafer 5 formed in this wayis shown in FIG. 10. Incidentally, FIG. 10 shows the front surface 2side of the sapphire substrate 1 with being turned upward in a mannersimilar to the drawing indicative of the process step PZ1.

As shown in FIG. 10, the edge portion 10 of the SOS wafer 5 is coveredwith the polysilicon layer 7 and the silicon nitride layer 9. Althoughthese layers are placed on the edge portions 10, they are unnecessaryaccording to the preferred embodiment of the prior art.

Since the ion implantation is effected only on the back surface 6,although the front surface 2 side of the sapphire substrate 1 is alsocovered with the polysilicon layer 7 and the silicon nitride layer 9,the region like the conductive diffusion region 8 on which the ionimplantation is effected, does not exist in the polysilicon layer 7 thatcovers the front surface 2 side and the edge portion 10.

The silicon nitride layer 9 and the polysilicon layer 7 formed on thefront surface 2 side of the sapphire substrate 1 according to theprocess steps PZ2 and PZ4 are removed by reactive ion etching so thatthe lower oxide layer 4 is exposed. Processing of the SOS wafer 5 placedin this state by the silicon wafer processing apparatus is enabled.

In the SOS wafer 5 formed in this way, the polysilicon layer 7 formed onthe back surface 6 side of the sapphire substrate 1 is non-transparentenough to detect the existence of the SOS wafer 5 by a photosensor. Thethickness of the polysilicon layer 7 is the suitable minimum thicknessbased on an indirect bandgap absorption method, enough to detect anon-transparent object by the photosensor.

Incidentally, it is necessary to change the thickness of the polysiliconlayer 7 to be grown, according to a photosensor system employed in thesilicon wafer processing apparatus. It is also necessary to change iteven depending upon the wavelength of used light. Red light and infraredradiation are used in the current silicon wafer processing apparatus andenough to optically detect the existence of the SOS wafer 5 according tothe thickness of the polysilicon layer 7 on the back surface 6 side.When light of other wavelength is used, the thickness of the polysiliconlayer 7 may be set to such a thickness that the SOS wafer 5 can bedetected as a silicon wafer, according to the photosensor employed inthe silicon wafer processing apparatus.

Further, in the SOS wafer 5 formed in the above-described manner, theconductive diffusion region 8 having conductivity obtained byion-implanting phosphorus into the polysilicon layer 7 is formed on theback surface 6 side.

The manufacturing method of the prior art does not form such acontamination level or defect density as to exert an influence on yieldon the front surface 2 side of the SOS wafer 5, in addition to the factthat optical characteristics and conductive characteristics are madeequivalent to the silicon wafer.

The films formed on the back surface 6 side of the SOS wafer 5 areadapted to a subsequent silicon processing process. This adaptability isobtained according to the selection of the films and the order ofdeposition of their films. That is, the polysilicon layer 7 and thesilicon nitride layer 9 are adapted to all subsequent heat treatments.

Further, the silicon nitride layer 9 corresponding to the outer filmserves as a diffusion barrier which prevents the silicon layer 3 on thefront surface 2 side of the SOS wafer 5 from being auto-doped with animpurity implanted in the back surface 6 side by diffusion thereofthrough the surface of nitride. Further, the silicon nitride layer 9corresponding to the outer film is inert to all acids except forphosphoric acid at a high temperature, and the phosphoric acid is notused even in any step.

A conventional SOS wafer has been manufactured by such a manufacturingmethod as described above (refer to, for example, a patent document 1(Japanese Patent No. 3083725 (paragraph 0011 in page 3—paragraph 0016 inpage 4, and FIGS. 1 through 5)).

There is known, as a technique for preventing warpage of an SOS wafer,one which forms a polysilicon layer in a back surface of a sapphiresubstrate and pulls back its warpage produced by a silicon layer on itsfront surface by virtue of the polysilicon layer formed in the backsurface (refer to, for example, Japanese Unexamined Patent PublicationNo. Sho 57 (1982)-153445 (lower right-hand section in page 2 and FIG.2)).

There is also known a technique which thins a polysilicon layer toprevent warpage of an SOS wafer (refer to, for example, JapaneseUnexamined Patent Publication No. 2000-36585 (paragraph 0015 in page 3and FIG. 1)).

However, the technique of the patent document 1 referred to above isaccompanied by problems that since silicon nitride layers are formed bylow-pressure chemical vapor deposition, and the silicon nitride layer onthe front surface side of a sapphire substrate is removed and thesilicon nitride layer is provided so as to exist only in its backsurface side, when they are used in processing of a silicon waferprocessing apparatus, the front surface of the sapphire substrate iswarped into convexity due to very large tensile stress developed in thesilicon nitride layer formed by the low-pressure chemical vapordeposition, thus causing a failure in wafer vacuum chuck on the backsurface side and a failure in transfer due to the warpage in asubsequent processing process of a semiconductor device, instability ina processing process for temperature ununiformity or the like at heattreatment, cracks of the SOS wafer, etc.

SUMMARY OF THE INVENTION

The present invention has been made to solve the above problems. It isan object of the present invention to provide means which preventswarpage of an SOS wafer whose processing is enabled in a normal siliconwafer processing apparatus.

According to one aspect of the present invention, for attaining theabove object, there is provided an SOS wafer comprising anon-transparent polysilicon layer provided on a back surface of asapphire substrate, a silicon nitride layer which protects thepolysilicon layer, and a stress relaxing film which cancels out stressproduced in the silicon nitride layer, wherein the silicon nitride layerand the stress relaxing film are provided on the back surface side.

Thus, the present invention brings about advantageous effects in thatwarpage and cracks of the SOS wafer in which the silicon nitride layeris provided on the back surface side of the sapphire substrate, can beprevented and the stability of a processing process of a semiconductordevice in the normal silicon wafer processing apparatus can be ensured.

BRIEF DESCRIPTION OF THE DRAWINGS

While the specification concludes with claims particularly pointing outand distinctly claiming the subject matter which is regarded as theinvention, it is believed that the invention, the objects and featuresof the invention and further objects, features and advantages thereofwill be better understood from the following description taken inconnection with the accompanying drawings in which:

FIG. 1 is an explanatory view showing a method for manufacturing an SOSwafer according to a first embodiment;

FIG. 2 is an explanatory view illustrating an edge portion of the SOSwafer according to the first embodiment;

FIG. 3 is an explanatory view depicting a method for manufacturing anSOS wafer according to a second embodiment;

FIG. 4 is an explanatory view illustrating an edge portion of the SOSwafer according to the second embodiment;

FIG. 5 is an explanatory view depicting a method for manufacturing anSOS wafer according to a third embodiment;

FIG. 6 is an explanatory view showing an edge portion of the SOS waferaccording to the third embodiment;

FIG. 7 is an explanatory view depicting a method for manufacturing anSOS wafer according to a fourth embodiment;

FIG. 8 is an explanatory view showing an edge portion of the SOS waferaccording to the fourth embodiment;

FIG. 9 is an explanatory view illustrating a method for manufacturing aconventional SOS wafer; and

FIG. 10 is an explanatory view showing an edge portion of theconventional SOS wafer.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of an SOS wafer according to the present inventionand its manufacturing method will hereinafter be described withreference to the accompanying drawings.

First Preferred Embodiment

FIG. 1 is an explanatory view showing a method for manufacturing an SOSwafer according to a first embodiment, and FIG. 2 is an explanatory viewillustrating an edge portion of the SOS wafer according to the firstembodiment, respectively.

Incidentally, constituent elements similar to those employed in theprior art are given the same reference numerals, and their explanationswill therefore be omitted.

In FIG. 1, reference numeral 13 indicates a silicon nitride film used asa stress relaxing film, which is deposited or grown by a plasma CVDmethod using a parallel plate type plasma CVD (Chemical VaporDeposition) apparatus.

A silicon layer 3 formed in a front surface 2 of a sapphire substrate 1employed in the present embodiment is a silicon layer monocrystallizedby epitaxial growth.

The method for manufacturing the SOS wafer according to the presentembodiment will be explained below in accordance with processes orprocess steps indicated by P using FIG. 1.

Incidentally, FIG. 1 shows partial cross-sections of the SOS wafer, andFIG. 2 shows the front surface side of the sapphire substrate 1 asdirected upward (this is similar in the drawings showing process stepsof the present invention shown below and the drawings showing edgeportions).

In P1, an SOS wafer 5 having a silicon layer 3 and an oxide layer 4laminated over the front surface 2 of the sapphire substrate 1 similarto one in the process or process step PZ1 of the prior art, is prepared.

In P2, a polysilicon layer 7 similar to one in the process step PZ2 ofthe prior art is formed on the back surface 6 of the sapphire substrate1 and the oxide layer 4 on the front surface 2 side thereof and at eachedge portion 10 by low-pressure chemical vapor deposition.

Incidentally, the drawing indicative of the present process step isshown with its obverse and reverse sides placed in reverse relationshipto the drawing indicative of the process step P1. The front surface 2side is not illustrated (the drawings indicative of the process steps ofthe present invention shown below are also same).

In P3, ions of an impurity for applying conductivity to the polysiliconlayer 7, such as phosphorus (P) are implanted into the polysilicon layer7 on the back surface 6 of the sapphire substrate 1 to form a conductivediffusion region 8 similar to one in the process step PZ3 of the priorart.

In P4, a 1550 Å-thick silicon nitride film 13 is formed on thepolysilicon layer 7 on the back surface 6 side of the sapphire substrate1 by the plasma CVD method.

An RF plasma power supply of the parallel plate type plasma CVDapparatus at this time is 13.56 MHz, and its deposition conditions are asusceptor temperature of 400° C., an SiH₄ flow rate of 105 sccm, an NH₃flow rate of 51 sccm, an N₂ flow rate of 1534 sccm, an RF power of 400W, a pressure of 5 Torr, and an electrode-to-electrode distance of 8.9mm.

Stress subsequent to the deposition of the silicon nitride film 13 grownunder such deposition conditions is 0.6 Gpa as compressive stress.

An Si—H concentration determined from an infrared absorption peak of aninfrared absorption spectrum of the silicon nitride film 13 is less thanor equal to 10×10²¹/cm³, and a low wavenumber-end wavenumber of an Si—Habsorption peak at infrared absorption is 1960/cm or more. Thus, thesilicon nitride film 13 satisfies a condition for eliminating aninelastic behavior described in Japanese Patent No. 3043083 (calleddocument 1), i.e., a condition for restoring the silicon nitride film 13to the original stress state upon cooling subsequent to execution ofthermal or heat treatment.

In P5, a 900 Å-thick silicon nitride layer 9 is formed on the frontsurface 2 side of the sapphire substrate 1 and over the polysiliconlayer 7 of the edge portion 10 and the silicon nitride film 13 on theback surface 6 side by low-pressure chemical vapor deposition.

The shape of each edge portion 10 of the SOS wafer 5 formed in this wayis configured as shown in FIG. 2. As shown in FIG. 2, the edge portion10 of the SOS wafer 5 is covered with the polysilicon layer 7 and thesilicon nitride layer 9. The polysilicon layer 7 on the front surface 2side of the sapphire substrate 1, and the polysilicon layer 7 and thesilicon nitride film 13 on the back surface 6 side are also covered withthe silicon nitride layer 9.

Thereafter, the silicon nitride layer 9 and the polysilicon layer 7formed on the front surface 2 side of the sapphire substrate 1 inaccordance with the process steps P2 and P5 are removed by reactive ionetching to expose the lower oxide layer 4. In this condition, theprocessing of the SOS wafer 5 according to the present embodiment by asilicon wafer processing apparatus is enabled.

The oxide film 4 is removed by a processing step at the silicon waferprocessing apparatus. In a backgrind process subsequent to the formationof a semiconductor device or the like in the silicon layer 3, thepolysilicon layer 7 on the back surface 6 side, the conductive diffusionregion 8 formed therein, the silicon nitride film 13 and the siliconnitride layer 9 are removed. The back surface 6 of the sapphiresubstrate 1 is exposed, followed by being divided into fractions,whereby a semiconductor device using the SOS wafer 5 according to themanufacturing method of the present embodiment is manufactured.

In the SOS wafer 5 formed by the manufacturing method of the presentembodiment, the conductive diffusion region 8 is formed in thepolysilicon layer 7 formed on the back surface 6 side of the sapphiresubstrate 1 to ensure non-transparency of the polysilicon layer 7. It istherefore possible to reliably detect the existence of the SOS wafer 5by a photosensor.

Tensile stress subsequent to the deposition of the 900 Å-thick siliconnitride layer 9 formed by the low-pressure chemical vapor deposition inthe process step P5 is 1 Gpa. Compressive stress subsequent to thedeposition of the 1550 Å-thick silicon nitride film 13 formed by theplasma CVD method in the process step P4 is 0.6 Gpa. Therefore, thetensile stress of the silicon nitride layer 9 can be cancelled out bythe compressive stress of the silicon nitride film 13 and hence nowarpage occurs in the SOS wafer 5.

Further, since the silicon nitride film 13 formed in the process step P4meets the condition of the document 1, the silicon nitride film 13 canbe restored to the original post-deposition compressive stress stateeven if subjected to thermal treatment. Even though the silicon nitridefilm 13 is subjected to a deposition temperature of 700 to 800° C. atwhich the silicon nitride layer 9 is deposited by the low-pressurechemical vapor deposition in the process step P5, no change occurs inthe compressive stress of the silicon nitride film 13 and no warpageoccurs in the SOS wafer 5 after its heat treatment. Incidentally, if thedeposition conditions described in the process step P4 are used, thenthe heat treatment of 700 to 800° C. causes no inelastic behavior.

Further, since the edge portion 10 and the polysilicon layer 7 andsilicon nitride film 13 on the back surface 6 side are covered with thesilicon nitride layer 9 inert to hydrofluoric acid, the polysiliconlayer 7 and the silicon nitride film 13 are not eroded by hydrofluoricacid processing included in a cleaning step or the like in theprocessing process of the semiconductor device. Further,non-transparency is not impaired with erosion of the conductivediffusion region 8.

Furthermore, since the edge portion 10 and the polysilicon layer 7 onthe back surface 6 side are covered with the silicon nitride layer 9, itis possible to prevent diffusion of impurities of the conductivediffusion region 8 into the silicon layer 3 on the front surface 2 sidedue to outward diffusion thereof. Further, no influence is exerted onthe characteristic of the semiconductor device formed in the siliconlayer 3.

In the present embodiment as described above, the tensile stress of thesilicon nitride layer is set so as to be cancelled by the compressivestress based on the silicon nitride film. It is thus possible to preventwarpage and cracks of the SOS wafer in which the silicon nitride layeris provided on the back surface side of the sapphire substrate and toensure the stability of the processing process of the semiconductordevice at the normal silicon wafer processing apparatus.

With the formation of the silicon nitride film by the plasma CVD method,the silicon nitride film that causes the compressive stress can easilybe formed, and the silicon nitride film can be grown only on the backsurface side of the sapphire substrate, thus making it possible toachieve efficiency-enhancing of the process step for removing thepolysilicon layer or the like on the front surface side of the sapphiresubstrate.

Further, since the silicon nitride film is deposited on the inelasticbehavior-eliminating condition, the stress state of the silicon nitridefilm can always be restored to the original state after cooling in theheat treating or annealing step done after the formation of the siliconnitride film, and hence warpage and cracks of the SOS wafer subsequentto the heat treatment can be prevented.

Second Preferred Embodiment

FIG. 3 is an explanatory view showing a method for manufacturing an SOSwafer according to a second embodiment, and FIG. 4 is an explanatoryview showing an edge portion of the SOS wafer according to the secondembodiment, respectively.

Incidentally, constituent elements similar to those employed in thefirst embodiment are given the same reference numerals, and theirexplanations will therefore be omitted.

The method for manufacturing the SOS wafer according to the presentembodiment will hereinafter be described in accordance with processes orprocess steps indicated by PA using FIG. 3.

In PA1, an SOS wafer 5 in which a silicon layer 3 and an oxide layer 4are laminated over a front surface 2 of a sapphire substrate 1, isprepared in a manner similar to the process step P1 of the firstembodiment.

In PA2, a polysilicon layer 7 is formed over a back surface 6 of thesapphire substrate 1 and the oxide layer 4 on the front surface 2 sideand at each edge portion 10 in a manner similar to the process step P2of the first embodiment.

In PA3, a conductive diffusion region 8 is formed in the polysiliconlayer 7 placed on the back surface 6 of the sapphire substrate 1 in amanner similar to the process step P3 of the first embodiment.

In PA4, a 900 Å-thick silicon nitride layer 9 is formed over each of thepolysilicon layers 7 on the front surface 2 side of the sapphiresubstrate 1, the back surface 6 side thereof and at the edge portions 10thereof by low-pressure chemical vapor deposition.

In PA5, a 1550 Å-thick silicon nitride film 13 is formed on the siliconnitride layer 9 on the back surface 6 side of the sapphire substrate 1on the conditions similar to the process step P4 of the first embodimentby a plasma CVD method.

The shape of the edge portion 10 of the SOS wafer 5 formed in this wayis configured as shown in FIG. 4. As shown in FIG. 4, the edge portion10 of the SOS wafer 5 is covered with the polysilicon layer 7 and thesilicon nitride layer 9. The polysilicon layers 7 on the front surface 2side and back surface 6 side of the sapphire substrate 1 are alsocovered with the silicon nitride layer 9, and a silicon nitride film 13is formed on the silicon nitride layer 9 on the back surface 6 side.

Thereafter, the silicon nitride layer 9 and the polysilicon layer 7formed on the front surface 2 side of the sapphire substrate 1 inaccordance with the process steps PA2 and PA4 are removed in a mannersimilar to the first embodiment. Thus, processing of the SOS wafer 5 ofthe present embodiment by a silicon wafer processing apparatus isenabled.

The removal of the oxide layer 4 and the removal or the like of thesilicon nitride film 13 or the like in a backgrind process are carriedout in a manner similar to the first embodiment, and a semiconductordevice using the SOS wafer 5 formed by the manufacturing method of thepresent embodiment is manufactured.

In the SOS wafer 5 formed by the manufacturing method of the presentembodiment, non-transparency similar to the first embodiment is ensuredat the polysilicon layer 7 on the back surface 6 side of the sapphiresubstrate 1. It is therefore possible to reliably detect the existenceof the SOS wafer 5 by a photosensor.

Tensile stress subsequent to the deposition of the 900 Å-thick siliconnitride layer 9 formed by the low-pressure chemical vapor deposition inthe process step PA4 is 1 Gpa. Compressive stress subsequent to thedeposition of the 1550 Å-thick silicon nitride film 13 formed by theplasma CVD method in the process step PA5 is 0.6 Gpa. Therefore, thetensile stress of the silicon nitride layer 9 can be cancelled out bythe compressive stress of the silicon nitride film 13 in a mannersimilar to the first embodiment, and hence no warpage occurs in the SOSwafer 5.

Since the silicon nitride film 13 is formed in the process step PA5after the formation of the silicon nitride layer 9 in the process stepPA4, the silicon nitride film 13 is insensitive to heat treatment at theformation of the silicon nitride layer 9 by the low-pressure chemicalvapor deposition. Incidentally, the silicon nitride film 13 is grown soas to meet the condition of the document 1 in the process step PA5because heat treatment in a subsequent process for processing asemiconductor device is taken into consideration. Since a restrictionbased on the condition of the document 1 is not placed if the heattreatment in the subsequent processing process is a relatively lowtemperature, the range for the condition of deposition of the siliconnitride film 13 can be enlarged.

Further, since the edge portion 10 and the polysilicon layer 7 on theback surface 6 side are covered with the silicon nitride layer 9 inertto hydrofluoric acid, the polysilicon layer 7 is not eroded byhydrofluoric acid processing, and non-transparency is not impaired witherosion of the conductive diffusion region 8.

Furthermore, since the edge portion 10 and the polysilicon layer 7 onthe back surface 6 side are covered with the silicon nitride layer 9,impurities of the conductive diffusion region 8 can be prevented frombeing diffused outward and hence no influence is exerted on thecharacteristic of the semiconductor device formed in the silicon layer3.

In the present embodiment as described above, the silicon nitride filmis formed after the formation of the silicon nitride layer in additionto an advantageous effect similar to the first embodiment. It is thuspossible to avoid the influence exerted on the silicon nitride film dueto the heat treatment at the formation of the silicon nitride layer.

Third Preferred Embodiment

FIG. 5 is an explanatory view showing a method for manufacturing an SOSwafer according to a third embodiment, and FIG. 6 is an explanatory viewshowing an edge portion of the SOS wafer according to the thirdembodiment, respectively.

Incidentally, constituent elements similar to those employed in thefirst embodiment are given the same reference numerals, and theirexplanations will therefore be omitted.

In FIG. 5, reference numeral 15 indicates a silicon oxide film used as astress relaxing film, which is deposited by a plasma CVD method using aparallel plate type plasma CVD apparatus.

The method for manufacturing the SOS wafer of the present embodimentwill be explained below in accordance with processes or process stepsindicated by PB using FIG. 5.

Since the process steps PB1 through PB3 of the present embodiment aresimilar to the process steps P1 through P3 of the first embodiment,their explanations are omitted.

In PB4, a 10000 Å-thick silicon oxide film 15 is formed on a polysiliconlayer 7 on the back surface 6 side of a sapphire substrate 1 by theplasma CVD method.

An RF plasma power supply of a parallel plate type plasma CVD apparatusat this time is 13.56 MHz, and its deposition conditions are as follows:a susceptor temperature of 400° C., a TEOS bubbling N₂ flow rate of 330sccm, an O₂ flow rate of 730 sccm, an RF power of 390 W, a pressure of10 Torr, and an electrode-to-electrode distance of 4.7 mm.

Stress subsequent to the deposition of the silicon oxide film 15 grownunder such deposition conditions is 0.13 Gpa as compressive stress.

An Si—H absorption coefficient determined from an infrared absorptionspectrum of the silicon oxide film 15 is less than or equal to1.5×10²/cm. The silicon oxide film 15 satisfies a condition under whichmoisture is not allowed to penetrate, which is described in JapaneseUnexamined Patent Publication No. Hei 5 (1993)-291411 (called a document2).

Incidentally, although the document 2 shows the invention related topermeability to moisture, it has been found out that when the siliconoxide film 15 is grown by reference to the deposition conditions of thepatent document 2 upon completion of the present invention, there is acorrelation even with respect to stress and the inelastic behavior ofthe silicon oxide film 15 due to heat treatment can be reduced.Therefore, the present deposition conditions were used as the conditionsfor restoring the silicon oxide film 15 to substantially the originalstress state upon cooling subsequent to execution of the heat treatment.

In PB5, a 900 Å-thick silicon nitride layer 9 is formed on the frontsurface 2 side of the sapphire substrate 1 and over the polysiliconlayer 7 of each edge portion 10 and the silicon oxide film 15 on theback surface 6 side of the sapphire substrate 1 by low-pressure chemicalvapor deposition.

The shape of each edge portion 10 of the SOS wafer 5 formed in this wayis configured as shown in FIG. 6. As shown in FIG. 6, the edge portion10 of the SOS wafer 5 is covered with the polysilicon layer 7 and thesilicon nitride layer 9. The polysilicon layer 7 on the front surface 2side of the sapphire substrate 1, and the polysilicon layer 7 andsilicon oxide film 15 on the back surface 6 side of the sapphiresubstrate 1 are also respectively covered with the silicon nitride layer9.

Thereafter, the silicon nitride layer 9 and the polysilicon layer 7formed on the front surface 2 side of the sapphire substrate 1 inaccordance with the process steps PB2 and PB5 are removed in a mannersimilar to the first embodiment. Thus, processing of the SOS wafer 5 ofthe present embodiment by a silicon wafer processing apparatus isenabled.

The removal of the oxide layer 4 and the removal or the like of thesilicon oxide film 15 or the like in a backgrind process are carried outin a manner similar to the first embodiment, and a semiconductor deviceusing the SOS wafer 5 formed by the manufacturing method of the presentembodiment is manufactured.

In the SOS wafer 5 formed by the manufacturing method of the presentembodiment, non-transparency similar to the first embodiment is ensuredat the polysilicon layer 7 on the back surface 6 side of the sapphiresubstrate 1. It is therefore possible to reliably detect the existenceof the SOS wafer 5 by a photosensor.

Tensile stress subsequent to the deposition of the 900 Å-thick siliconnitride layer 9 formed by the low-pressure chemical vapor deposition inthe process step PB5 is lGpa. Compressive stress subsequent to thedeposition of the 10000 Å-thick silicon oxide film 15 formed by theplasma CVD method in the process step PB4 is 0.13 Gpa. Therefore, stressexceeding the tensile stress of the silicon nitride layer 9, which iscancelled out by the compressive stress of the silicon oxide film 15, isreached. Since, however, the inelastic behavior remains although smallas described above, the compressive stress is reduced from 0.13 Gpa to0.1 Gpa when the silicon oxide film is subjected to a depositiontemperature of 700 to 800° C. at which the silicon nitride layer isdeposited by the low-pressure chemical vapor deposition in the processstep PB5. It is therefore possible to cancel the tensile stress of thesilicon nitride layer 9 by the post-cooling compression stress in theheat treatment of the process step PA5.

Thus, when the inelastic behavior remains in the compressive stress, thetensile stress and compressive stress produced by the silicon nitridelayer 9 and the silicon oxide film 15 can be matched with each otherafter heat treatment if the deposition conditions are set in expectationof its change, and hence no warpage occurs in the SOS wafer 5.

Further, since each edge portion 10 and the polysilicon layer 7 on theback surface 6 side are covered with the silicon nitride layer 9 inertto hydrofluoric acid, the polysilicon layer 7 is not eroded byhydrofluoric acid processing and non-transparency is not impaired witherosion of the conductive diffusion region 8.

Further, since the edge portion 10 and the polysilicon layer 7 on theback surface 6 side are covered with the silicon nitride layer 9,outward diffusion of impurities of the conductive diffusion region 8 canbe prevented and hence no influence is exerted on the characteristic ofa semiconductor device formed in a silicon layer 3.

In the present embodiment as described above, an advantageous effectsimilar to the first embodiment can be obtained even when the siliconoxide film is used.

That is, the tensile stress of the silicon nitride layer is cancelled bythe compressive stress produced by the silicon oxide film, after theformation of the silicon nitride layer. It is thus possible to preventwarpage and cracks of the SOS wafer in which the silicon nitride layeris provided on the back surface side of the sapphire substrate and toensure the stability of the processing process of the semiconductordevice at the normal silicon wafer processing apparatus.

With the formation of the silicon oxide film by the plasma CVD method,the silicon oxide film that causes the compressive stress can easily beformed, and the silicon oxide film can be grown only on the back surfaceside of the sapphire substrate, thus making it possible to achieveefficiency-enhancing of the process step for removing the polysiliconlayer or the like on the front surface side of the sapphire substrate.

Further, since the silicon oxide film is deposited on the inelasticbehavior-reducing condition, a change in stress of the silicon oxidefilm, which is developed after cooling in the heat treating step carriedout after the formation of the silicon oxide film, can easily beexpected. Warpage and cracks of the SOS wafer subsequent to the heattreatment can be prevented.

Fourth Preferred Embodiment

FIG. 7 is an explanatory view showing a method for manufacturing an SOSwafer according to a fourth embodiment, and FIG. 8 is an explanatoryview showing an edge portion of the SOS wafer according to the fourthembodiment, respectively.

Incidentally, constituent elements similar to those employed in thethird embodiment are given the same reference numerals, and theirexplanations will therefore be omitted.

The method for manufacturing the SOS wafer according to the presentembodiment will be explained below in accordance with processes orprocess steps indicated by PC using FIG. 7.

Since the process steps PC1 through PC4 of the present embodiment aresimilar to the process steps PA1 through PA4 of the second embodiment,their explanations are omitted.

In PC5, a 8000 Å-thick silicon oxide film 15 is formed on a siliconnitride layer 9 on the back surface 6 side of a sapphire substrate 1 bya plasma CVD method on the condition similar to the process step PB4 ofthe third embodiment.

The shape of each edge portion 10 of the SOS wafer 5 formed in this wayis configured as shown in FIG. 8. As shown in FIG. 8, the edge portion10 of the SOS wafer 5 is covered with a polysilicon layer 7 and thesilicon nitride layer 9. The polysilicon layers 7 on the front surface 2side and back surface 6 side of the sapphire substrate 1 are alsocovered with the silicon nitride layer 9, and a silicon oxide film 15 isformed on the silicon nitride layer 9 on the back surface 6 side.

Thereafter, the silicon nitride layer 9 and the polysilicon layer 7formed on the front surface 2 side of the sapphire substrate 1 inaccordance with the process steps PC2 and PC4 are removed in a mannersimilar to the first embodiment. Thus, processing of the SOS wafer 5 ofthe present embodiment by a silicon wafer processing apparatus isenabled.

The removal of an oxide layer 4 and the removal or the like of thesilicon oxide film 15 or the like in a backgrind process are carried outin a manner similar to the first embodiment, and a semiconductor deviceusing the SOS wafer 5 formed by the manufacturing method of the presentembodiment is manufactured.

In the SOS wafer 5 formed by the manufacturing method of the presentembodiment, non-transparency similar to the first embodiment is ensuredat the polysilicon layer 7 on the back surface 6 side of the sapphiresubstrate 1. It is therefore possible to reliably detect the existenceof the SOS wafer 5 by a photosensor.

Tensile stress subsequent to the deposition of the 900 Å-thick siliconnitride layer 9 formed by low-pressure chemical vapor deposition in theprocess step PC4 is 1 Gpa. Compressive stress subsequent to thedeposition of the 8000 Å-thick silicon oxide film 15 formed by theplasma CVD method in the process step PC5 is 0.13 Gpa. Therefore, thetensile stress of the silicon nitride layer 9 can be cancelled out bythe compressive stress of the silicon oxide film 15 in a manner similarto the first embodiment, and hence no warpage occurs in the SOS wafer 5.

Since the silicon oxide film 15 is formed in the process step PC5 afterthe formation of the silicon nitride layer 9 in the process step PC4,the silicon oxide film 15 is insensitive to heat treatment at theformation of the silicon nitride layer 9 by the low-pressure chemicalvapor deposition. Therefore, the thickness of the silicon oxide film 15was made thin like 8000 Å. Incidentally, the silicon oxide film 15 isgrown so as to meet the condition of the document 2 in the process stepPC5 because heat treatment in a subsequent process for processing asemiconductor device is taken into consideration. Since a restrictionbased on the condition of the document 2 is not placed if the heattreatment in the subsequent processing process is placed at a relativelylow temperature, the range for the condition of deposition of thesilicon oxide film 15 can be enlarged.

Further, since the edge portion 10 and the polysilicon layer 7 on theback surface 6 side are covered with the silicon nitride layer 9 inertto hydrofluoric acid, the polysilicon layer 7 is not eroded byhydrofluoric acid processing, and non-transparency is not impaired witherosion of a conductive diffusion region 8.

Furthermore, since the edge portion 10 and the polysilicon layer 7 onthe back surface 6 side are covered with the silicon nitride layer 9,outward diffusion of impurities of the conductive diffusion region 8 canbe prevented and hence no influence is exerted on the characteristic ofa semiconductor device formed in a silicon layer 3.

Incidentally, since the etching rate of the silicon oxide film 15 withrespect to hydrofluoric acid is relatively large as compared with thesilicon nitride film 13, a reduction in the thickness due to thehydrofluoric acid processing included in a subsequent cleaning step orthe like must be taken into consideration. However, in consideration ofthe fact that a step for making the stability of the SOS wafer 5important exists before the first hydrofluoric acid processing in asubsequent processing process, priority is given to a reduction in theamount of warpage in that step, and the thickness of the silicon oxidefilm 15 was set to 8000 Å.

In the present embodiment as described above, the silicon oxide film isformed after the formation of the silicon nitride layer in addition toan advantageous effect similar to the third embodiment. It is thuspossible to avoid the influence of heat treatment at the formation ofthe silicon nitride layer on the silicon oxide film.

Incidentally, although each of the above embodiments has explained byway of example, the specific thicknesses of the silicon nitride film andsilicon oxide film as the stress relaxing films necessary to cancel thetensile stress of the silicon nitride layer, the thicknesses of thesilicon nitride film and the silicon oxide film need to be changedaccording to the thickness of the silicon nitride layer and the tensilestress produced therein. That is, it is important that the thickness ofeach of the silicon nitride film and the silicon oxide film isdetermined on the basis of the compressive stress developed in each ofthe silicon nitride film and silicon oxide film formed as the stressrelaxing films, so as to reach the compressive force or stress thatbecomes equivalent to the tensile force or stress obtained according tothe tensile stress and thickness of the silicon nitride layer.

Although each of the above embodiments has explained as an example, thecase in which the silicon nitride layer produces the tensile stress, asimilar effect can be obtained if the silicon nitride film or the likeused as the stress relaxing film that produces the tensile stress isused, similarly even where the silicon nitride layer produces thecompressive stress.

While the preferred forms of the present invention have been described,it is to be understood that modifications will be apparent to thoseskilled in the art without departing from the spirit of the invention.The scope of the invention is to be determined solely by the followingclaims.

1. A method for manufacturing an SOS wafer provided with a sapphiresubstrate, comprising the steps of: forming a polysilicon layer over thesapphire substrate; implanting an impurity for applying conductivity ina polysilicon layer on a back surface of the sapphire substrate to forma conductive diffusion region; forming a silicon nitride layer forprotecting the polysilicon layer, on the back surface side of thesapphire substrate; and forming a stress relaxing film for cancelingstress of the silicon nitride layer, on the back surface side of thesapphire substrate.
 2. The method according to claim 1, wherein saidstress relaxing film forming step is performed prior to said siliconnitride layer forming step.
 3. The method according to claim 2, whereinwhen the silicon nitride layer is in a tensile stress state, said stressrelaxing film forming step is set as a step for forming a siliconnitride film by a plasma CVD method.
 4. The method according to claim 3,wherein an Si—H concentration determined from an infrared absorptionpeak of the silicon nitride film is less than or equal to 10×10²¹/cm³,and a low wavenumber-end wavenumber of an Si—H absorption peak atinfrared absorption of the silicon nitride film is greater than or equalto 1960/cm.
 5. The method according to claim 1, wherein said stressrelaxing film forming step is performed after said silicon nitride layerforming step.
 6. The method according to claim 5, wherein when thesilicon nitride layer is in a tensile stress state, said stress relaxingfilm forming step is set as a step for forming a silicon nitride film bya plasma CVD method.
 7. The method according to claim 6, wherein an Si—Hconcentration determined from an infrared absorption peak of the siliconnitride film is less than or equal to 10×10²¹/cm³, and a lowwavenumber-end wavenumber of an Si—H absorption peak at infraredabsorption of the silicon nitride film is greater than or equal to1960/cm.
 8. The method according to claim 2, wherein when the siliconnitride layer is in a tensile stress state, said stress relaxing filmforming step is set as a step for forming a silicon oxide film by aplasma CVD method.
 9. The method according to claim 5, wherein when thesilicon nitride layer is in a tensile stress state, said stress relaxingfilm forming step is set as a step for forming a silicon oxide film by aplasma CVD method.